1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method for manufacturing MOSFET.
2. Background of the Related Art
Generally, with high packing density of a semiconductor device, there has been provided a method for simultaneously manufacturing devices having different functions or a method for forming a dual gate having gate insulating films of different thicknesses.
In manufacturing a device having gate insulating films of different thicknesses, it is most preferable that both a device having a thin gate insulating film and a device having a thick gate insulating film have desired characteristics at the same time.
A related art method for manufacturing a semiconductor device will be described with reference t o the accompanying drawings .
FIGS. 1a to 1d are sectional views illustrating a related art method for manufacturing a semiconductor device.
As shown in FIG. 1a, dual gate insulating films 13 and 13a are formed by a typical dual gate oxidation process and then gate electrodes 14 and 14a are formed. That is to say, a gate electrode 14 having a thin gate insulating film 13 and a gate electrode 14a having a relatively thick gate insulating film 13a are formed on a semiconductor substrate 11. A reference numeral 12 which is not described denotes a device isolation film.
Afterwards, as shown in FIG. 1b, lightly doped drain (LDD) regions 15 and 15a are formed into the semiconductor substrate 11 by low-concentration impurity ion implantation using the gate electrodes 14 and 14a as masks.
As shown in FIG. 1c, an insulating film is deposited on an entire surface of the semiconductor substrate 11 including the gate electrodes 14 and 14a. The insulating film is then etched back to form sidewall spacers 16 and 16a at both sides of the gate electrodes 14 and 14a.
As shown in FIG. 1d, source/drain impurity regions 17 and 17a are formed by high-concentration impurity ion implantation using the gate electrodes 14 and 14a and the sidewall spacers 16 and 16a as masks. As a result, the related art method for manufacturing a semiconductor device is completed.
However, the related art method for manufacturing a semiconductor device has several problems.
In case that the thin gate insulating film and the thick gate insulating film are formed at the same time, hot carrier life time characteristic becomes poorer in the device having the thick gate insulating film than the device having the thin gate insulating film, thereby reducing reliability of the device.